Free radial photography in which an image-forming material is imagewise exposed to light to visualize the exposed area has been used in many photographic applications.
In a particularly useful photographic image formation system, various leuco dyes are reacted with a photo-oxidizing agent to oxidatively develop a color as described, e.g., in Photo. Sci. Eng., Vol. 5, 98-103 (1961), JP-B-43-29407 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-A-55-55335 (corresponding to U.S. Pat. No. 4,271,251), JP-A-57-60329 (corresponding to U.S. Pat. No. 4,298,678) , and JP-A-62-66254 (corresponding to U.S. Pat. No. 4,622,286) (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
These image-forming materials are difficult to handle because they may undergo an undesired color formation reaction, even after the imagewise exposure to light, when exposed to ordinary room light, sun light or white light.
Once an image is formed in the imagewise exposed area, color formation in the unexposed area should be prevented in order to keep the original image. For example, it is known that an original image can be maintained by applying a solution of a reducing agent such as a free radical scavenger (e.g., hydroquinone) to the material by spraying or impregnation after image formation. Although this technique is advantageous for maintenance or fixation of the image, the wet process involved therein inevitably makes the operation complicated.
There are on the market materials which can be processed only with light, for example, "Dylux" (a registered trade name of E. I. DuPont). The materials of this type form an image on exposure to UV light, and the image is then fixed by activation of a photo-reducing substance by visible light. According to this system, however, since light is used twice, the processor must be occupied for the amount of time it takes for light exposure, and exchanges of spectral filters are required. These requirements cause the processing speed to be unsatisfactory for some usages.
JP-B-43-29407 cited above discloses that imagewise exposure can be followed by heat fixation with a reducing heat-fixing agent which has been incorporated into a binder solution together with the leuco dye and photo-oxidizing agent or coated on a photosensitive layer. However, the fact that the fixing agent exists close to the photosensitive part (i.e., the leuco dye and photo-oxidizing agent) undesirably causes deterioration of sensitivity with time.
On the other hand, the image-forming materials containing the leuco dye and photo-oxidizing agent are usually prepared by uniformly dissolving the leuco dye and photo-oxidizing agent in an organic solvent, applying the solution on a support such as paper or plastic film by coating, immersion, casting or similar techniques, and then removing the solvent by drying. Use of such a volatile organic solvent requires the apparatus to have special safety features to guard against danger of explosion, and is hence disadvantageous with respect to safety and cost.